test 2291.1: misleading TRUELENGTH now impossible

Now that data.table objects have the GROWABLE_BIT set, R will reset TRUELENGTH
when duplicating them, causing our code to take a different branch.

Author: aitap

inst/tests/tests.Rraw

@@ -19453,7 +19453,6 @@ test(2290.4, DT[, `:=`(a = 2, c := 3)], error="It looks like you re-used `:=` in
 df = data.frame(a=1:3)
 setDT(df)
 attr(df, "att") = 1
-##test(2291.1, set(df, NULL, "new", "new"), error="attributes .* have been reassigned")
 test(2291.1, set(df, NULL, "new", "new"), error="either been loaded from disk.*or constructed manually.*Please run setDT.*setalloccol.*on it first")
 
 # ns-qualified bysub error, #6493